The assessment of energy expenditure in free-living subjects is central to a complete understanding of the etiology of obesity, malnutrition, coronary heart disease, and osteoporosis. Laboratory-based methods that rate high with respect to validity and reliability do not lend themselves to this task because they are restrictive, expensive, or both. Investigators have therefore developed survey methods, physiological markers, and mechanical or electrical monitors for use in the field. The development of the doubly labeled water method for measuring energy expenditure and increased availability of room indirect calorimeters has recently made it possible to evaluate these field techniques. Some of the recently developed mechanical and electrical monitors have been found to be valid for the measurement of energy expenditure, but even the best provide measures that are too variable to be useful on an individual basis.